Security has been tightened at North Korea’s Sohae satellite-launching station — changes that a U.S. think tank says could indicate the site might soon be occupied by space-agency scientists, engineers, technicians and the military.

Sohae, near Tongchang-ri in the communist country’s hilly northwest, was used in February to successfully launch a long-range rocket that put a satellite into orbit, though critics say the real purpose was to test ballistic-missile technology banned by the United Nations.

Recent satellite images show the site’s inner and outer security perimeters have been extended in recent years, according to an analysis by 38 North, a website run by Johns Hopkins University’s School of Advanced International Studies that monitors North Korean activities.

What once consisted of simple patrol paths atop surrounding hills now includes fences and new guard positions, the website said. Coastal areas have been connected to existing coastal-defense patrol roads.

The security upgrades, while likely tied to the site’s long-term construction plan, could also “reflect a growing North Korean concern of intelligence collection by foreign governments using defectors from the area or outside agents to infiltrate and collect information,” the analysis said.

February’s launch from Sohae — along with the North’s fourth underground nuclear test a month earlier — led to a U.S.-South Korean agreement to station a Terminal High-Altitude Area Defense system, known as THAAD, on the peninsula.

On Friday, the U.N. Security Council called recent North Korean ballistic-missile launches, including one fired from a submarine last week, “grave violations” of its ban on all ballistic-missile activity, The Associated Press reported. The council also urged its member states to “redouble their efforts” in implementing harsher sanctions against Pyongyang that were imposed in the spring.

The latest launch came days after the U.S. and South Korea started annual Ulchi Freedom Guardian war games seen by the North as a rehearsal for invasion.

Work is also continuing at the North’s Punggye-ri underground nuclear test site, particularly at the north portal where the North claimed to have detonated a hydrogen bomb in January, 38 North reported last month.

Satellite images from August show a large canopy erected south of a support building that blocks overhead observation near the test tunnel’s entrance, the website said. Supplies, equipment and vehicles spotted near the portal in images from July were no longer present.

“The purpose of the activity as well as of an object located on the tailings pile to the east of the North Portal is unclear,” the analysis said.

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This photo from the Korean Central News Agency purports to show the launch of a long-range ballistic missile from North Korea's Sohae satellite-launching station in February 2016.COURTESY OF KCNA